GEOLOGY of OLD CROW BASIN

"Long ago some men tried to kill a giant. The giant climbed up a mountain. The men climbed up too. The giant grabbed the two men and jumped over the mountain and got killed too. The giant is under the mountain in Old Crow." Gerald Nukon, Old Crow (Council for Yukon Indians 1981)

"People always like to protect mountains. They're formidable. We can't move them. We can't destroy them. They're special to us." Darius Kassi (Sherry and Vuntut Gwitchin First Nation 1999).

INTRODUCTION

The layers of rock that form the mountains, valleys, and basins of the northern Yukon (Figure 3.1) originated as layers of sediments formed at the bottom of ancient seas some 600 million years ago. After pressure and heat transformed these sediments into layers of rock, they were shifted, lifted, folded, and deformed by the movements of the earth's crust in periods of mountain building from about 400 to 60 million years ago.

The Old Crow Basin is a huge bowl-shaped structural depression that formed some 60 million years ago as a result of the final phase of mountain building in this region. The rim of the Basin is formed by the summits of the surrounding mountains: on the east are the Richardson Mountains, on the north, the British Mountains, and on the west and south, the Old Crow Mountains (Figure 3.2). The floor of the Basin is known as the Old Crow Flats. The sides of the Basin are the gentle slopes between the Flats and the mountains which rise 600 m to 1200 m above the Flats. Low-lying passes can also be found in any of the surrounding mountain ranges. Underlying the vegetation and the lakes and ponds of the Flats are many layers of sediments. The thin top layer is of relatively recent sediments; clay, silt, and sand. Under this layer are thick sequences of much older sedimentary rocks. Studies suggest that these older layers may be up to 4000 m thick.

The story of how this basin is thought to have formed, and how the study of the present-day mountains, rocks, and rock formations have led to and supported the geologists' theories, is the framework of this chapter.

The story of the last two million years and how the rocks and landscape have been affected by the forces of weathering, erosion, and the great Ice Ages is the focus of "Geomorphology". As one of the few areas in Canada untouched by glaciation during the Pleistocene Ice Ages, the Old Crow Basin area was a refuge for many ice-age plants and animals. Hundreds of thousands of fossil remnants of ice-age plants and animals were preserved in the layers of silt and clay. These fossils and the past environments they represent are discussed in " Palaeontology". The fossils of organisms found in the layers of the much older sedimentary rocks are also treated in more detail in "Paleontology".